021025 CHEM 102 SP25 Unit 5 pre

Unit 5

• Factors that Affect Solubility

Recall: Solution Formation

• Solutions form based on the intermolecular forces of the solute and solvent.

• Like dissolves like!

  • Ionic compounds dissolve in water due to ion-dipole forces.

  • Polar compounds dissolve in polar solvents.

  • Nonpolar compounds dissolve in nonpolar solvents.

Enthalpy of Solution

• The change in enthalpy upon solution formation is called the enthalpy of solution (ΔH° sol’n).

• Consider three steps in the process of forming a solution:

  1. Separate the solute

     • ΔH determined by lattice energy (ionic compounds) and intermolecular forces (covalent compounds).

     • This process is endothermic (ΔH > 0).

  2. Separate the solvent

  • ΔH is determined by intermolecular forces.

  • This step is also endothermic (ΔH > 0).

  3. Combine the solute and solvent particles

     • This process is exothermic (ΔH < 0).

• Example: KF(s) → KF(aq)

  • ΔH° sol’n = -17.7 kJ

Energy and Solubility: Exothermic Conditions

• Conditions when ΔH° sol’n < 0 (exothermic process):

  • ΔHsolute separation < 0

  • ΔHsolvent separation < 0

  • ΔHsolvation > 0

• Equation:

  • ∆𝐻𝑠𝑜𝑙𝑣𝑎𝑡𝑖𝑜𝑛 = ∆𝐻𝑒𝑓𝑓 − ∆𝐻𝑖𝑛 < 0

• The overall enthalpy change during solution formation is negative, indicating that the release of energy occurs.

Energy and Solubility: Endothermic Conditions

• Conditions when ΔH° sol’n > 0 (endothermic process):

  • ΔHsolute separation < 0

  • ΔHsolvent separation < 0

  • ΔHsolvation > 0

• Equation:

  • ∆𝐻𝑠𝑜𝑙𝑣𝑎𝑡𝑖𝑜𝑛 = ∆𝐻𝑒𝑓𝑓 − ∆𝐻𝑖𝑛 > 0

• The overall enthalpy change is positive, indicating absorption of energy during solution formation.

Effect of Temperature and Pressure on Solubility

• SOLIDS

  • For nearly all solids, as temperature increases, solubility increases.

  • Pressure does not affect the solubility of solids.

• GASES

  • For gases, as temperature increases, solubility decreases.

  • As pressure increases, solubility increases.

Henry’s Law – Pressure and Gas Solubility

• When a gas solute is present, molecules can enter the solvent to form a solution.

• The molar solubility of the gas (S_gas) is directly proportional to the pressure of the solute gas (P_gas) above the solvent.

  • Formula:

    • S_gas = k_H * P_gas

      • Where k_H is Henry's constant, determined experimentally for each gas/solvent combination.

• Equation representation:

  • (S₁/P₁) = (S₂/P₂)